This invention relates to capacitive fluid-gauging systems.
The invention is more particularly, but not exclusively concerned with systems for gauging the contents of an aircraft fuel tank.
Capacitive fuel-gauging probes used in aircraft fuel tanks commonly have an outer tubular electrode within which extends coaxially an inner rod-like electrode. The probe is mounted vertically within the tank and is open to allow any fuel within the tank to fill the space between the electrodes, to the same height as the fuel in the tank. As the height of fuel within the probe rises and falls, with change in the quantity of fuel in the tank, the capacitance between the two electrodes varies accordingly.
A measure of the dielectric constant of the fuel is obtained so that the mass of fuel can be determined regardless of variations in dielectric constant arising from temperature change or the use of different fuels. The dielectric constant is ususally determined by means of a permittivity sensor or so-called K-cell mounted at the lower end of the probe so that it is always immersed in any fuel that may be present.
In an ideal situation the capacitance of the probe is linearly related to the height of fuel in the tank. However, in practice, the fuel in the tank does not have identical properties at different depths. Typically, a temperature gradient will exist in the fuel causing its dielectric constant to vary with height. The use of a single K-cell mounted at the bottom of the tank will not therefore provide compensation for the variations in dielectric constant throughout the fuel.
Without the use of a K-cell, the probe measures over its full height and therefore the stratifications in the fuel tend to be cancelled and the probe output is dependent on the average dielectric constant of the fuel. This results in a probe output that is related in a non-linear fashion to fuel quantity.
In an attempt to make this output more linear, that is, to achieve full-height compensation, previous arrangements have used an additional capacitor connected in series with the probe. This, however, has various disadvantages. Firstly, it is expensive to provide capacitors that have a low temperature coefficient. Secondly, capacitors are usually only available with a small number of different values. Also, capacitors have the disadvantage that they produce a large attenuation in the output of the probe.